In binocular vision, the lateral displacement of the eyes gives
rise to both horizontal and vertical disparities between the
images projected onto the left and right retinae. While it is
well known that horizontal disparity is exploited by the binocular
visual system of birds and mammals to enable depth perception,
the role of vertical disparity is still largely unclear. In
this study, neuronal activity in the visual forebrain (visual
Wulst) of behaving barn owls to vertical disparity was
investigated. Single-unit responses to global random-dot
stereograms (RDS) were recorded with chronically implanted
electrodes and transmitted via radiotelemetry. Nearly
half of the cells investigated (44%, 16/36) varied the discharge
as a function of vertical disparity. Like horizontal-disparity
tuning profiles, vertical-disparity tuning curves typically
exhibited periodic modulation with side peaks flanking a prominent
main peak, and thus, could be fitted well with a Gabor function.
This indicates that tuning to vertical disparity was not caused
by disrupting horizontal-disparity tuning via vertical
stimulus offset, but by classical disparity detectors whose
orientation tuning was tilted. When tested with horizontal in
addition to vertical disparity, almost all cells investigated
(92%, 12/13) were tuned to both kinds of disparity. The emergence
of disparity detectors sensitive in two dimensions (horizontal
and vertical) is discussed within the framework of the disparity
energy model.